SEOUL, South Korea (CNN) -- It's 10.30 on Friday night, and scientists and doctors prepare to enter a veterinary hospital operating room at Seoul National University.

CNN has been invited by one of South Korea's leading cloning experts, Dr. Hwang Woo-Suk, to witness a rare event in genetic science, the birth of "humanized" pigs.

These genetically modified animals' organs have been tailor-made so that the size of them will allow them to be transplanted into humans.

Pig heart valves are widely used to patch up human hearts but one of the biggest problems in using the entire organ is ensuring the human body doesn't reject it.

The process to deliver the tailor-made piglets takes less than five minutes.

As the impregnated pig is sedated, her entire uterus is removed and placed in a sanitary incubator so that the baby pigs will be germ-free as soon as they are born.

The clean environment is important in order to prevent pig diseases being transferred to human organs.

Everyone in the room holds their breath to see if the piglets start breathing. They do.

The project is one of many high-profile projects Hwang has been involved with.

In a laboratory close to the operating theatre, meanwhile, doctors are artificially severing the spinal cord of rats, which will undergo stem cell therapy.

Last February, Hwang's research made headlines around the world when he obtained embryonic stem cells from a cloned human embryo.

From the embryos, he extracted stem cell lines -- a collection of master cells -- which can differentiate to any tissue in the body, potentially allowing scientists to one day grow replacement tissues to treat spinal cord injuries as well as diabetes and Parkinson's disease.

But tonight's humanized piglets are a different line of research, one that could produce quicker results.

The process starts in a sterile laboratory, where an army of researchers extract pig and cow eggs, then genetically alters them to produce organs that will not be rejected by humans.

Then three times a week, the cultured eggs are sped from the lab to a pig farm outside Seoul, and Hwang implants the eggs into a surrogate pig.

They have a 30 percent chance of carrying the fetus to full term. He has conducted about 1,500 procedures so far.

Dr. Hwang gets hundreds of encouraging e-mails and phone calls from patients suffering from such diseases, but he also gets his share of critics.

Religious groups say it is unethical because the research destroys an embryo that could develop into a human being.

But Hwang disagrees.

"I hope the standard of ethics will change with the development of technology," he told CNN.

Hwang's work has been widely welcomed by the South Korean Government, who has jumped on board by recently providing him with official approval for stem-cell research. It wants Hwang's work to help the country become a leader in this field.

"I can imagine five or 10 years from now, transplanting the organs of these pigs after they grow up into people who are in critical need of hearts," cardiovascular surgeon Dr. Lee Jong-Ryul told CNN.

Hwang has already successfully transplanted pig hearts into dogs.

He told CNN that the same will be performed on monkeys later this year, and projects that successful organ transplants from pigs to humans is not far off.